1. Field of the Invention
The present invention relates to a polyimide sheet prepared from a specific polyimide and a preparation process for the polyimide sheet. More particularly, the invention relates to a polyimide sheet prepared from a polyimide which can be processed by melt-extrusion and the process for preparing a polyimide sheet by melt-extrusion.
2. Prior Art of the Invention
Aromatic polyimide has excellent mechanical properties, solvent resistance and electrical insulative properties in addition to the highest thermal resistance among organic polymers. Examples of the aromatic polyimide are a polyimide prepared from bis(4-aminophenyl)ether and pyromellitic dianhydride (KAPTON,VESPEL; a product of E. I. Du Pont de Nemours & Co.) and a polyimide prepared from bis(4-aminophenyl) ether and 3,3',4,4'-biphenyltetracarboxylic dianhydride (UBILEX; a product of Ube Industries Co.).
Since heat-melting is difficult in these polyimides, these polyimides are prepared by initially casting a polyamic acid precursor into a sheet form and subsequently carrying out a heat imidization reaction. The preparation process has a rate-determining stage in the solvent removal step and the imidization step, is poor in productivity and further requires a solvent recovery step. Consequently, the process is economically unfavorable.
As another polyimide, a polyetherimide having thermoplasticity has been developed (ULTEM; a product of General Electric(GE)).
The polyetherimide having thermoplasticity can be used to prepare a sheet by the melt-extrusion process.
However, the sheet obtained by using the polyetherimide is unsatisfactory in view of heat and chemical resistance. Consequently, restrictions have been imposed upon the fields of application.
Another polyimide capable of being melted by heating has been disclosed in Japanese Patent Laid-Open Nos.62-68817 and 62-205124. However, a preparation process of a sheet by melt-extrusion process is not disclosed.